As mobile phones and hands-free telephone calls in an automobile have been widely used, there has been a demand for noise suppression performed at the time of calling under a noise environment. For example, under a noise environment in which stationary noise, such as road noise, and the like, is large, there is a desire for a technique for increasing a noise suppression amount and thus making voice be easily heard. Therefore, there have been attempts to perform noise suppression with less voice distortion on voice data under a noise environment.
For example, there is known a technique for estimating a target value that indicates a level to which the noise is suppressed, based on a representative value of signals obtained by transforming a signal of voice including noise for a predetermined period of time from a time area to a frequency area. There is also another known technique in which a coefficient used for noise suppression is calculated based on an amplitude component of voice for each predetermined frequency band, and the calculated coefficient is multiplied on a signal on the frequency axis of the original signal, thereby suppressing noise. For noise suppression, a technique for controlling upper and lower limits of noise suppression and a technique for correcting a coefficient depending on whether a signal seems to be voice or non-voice are also known (see, for example, International Publication Pamphlet No. WO2012/098579, Japanese Laid-open Patent Publication No. 2001-267973, Japanese Laid-open Patent Publication No. 2010-204392, and Japanese Laid-open Patent Publication No. 2007-183306).
As a related technique, a technique in which whether a plurality of frames having a predetermined length, which are obtained from a voice signal, are voice frames or non-voice frames is determined and a non-stationary frame is detected based on a non-stationary condition that indicates a non-voice frame is non-stationary is known (see, for example, Japanese Laid-open Patent Publication No. 2010-230814).